Method for casting molten metal, apparatus for the same, and cast slab

ABSTRACT

The present invention provides a continuous casting method in which vibration is given to molten metal by a shifting magnetic field so that the equi-axed crystal ratio can be enhanced and the equi-axed crystals can be made fine without generating surface defects caused by powder trapping. Further, the present invention provides an apparatus to which the continuous casting method is applied. Furthermore, the present invention provides a cast slab produced by the above method and apparatus. The method of casting molten metal comprises the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is accelerated by a high intensity and a low intensity of acceleration in a range not exceeding a predetermined flow velocity when the directional vectors of high acceleration and low acceleration in the same direction or in the opposite direction are combined with each other.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of casting molten steelwhen molten steel is vibrated by the action of an electromagnetic coil.Also the present invention relates to a continuous casting apparatus forthe method of casting molten steel and a cast slab which has been castby the method and the apparatus. More particularly, the presentinvention relates to a method of casting molten steel, an apparatus forthe method of casting molten steel and a cast slab which has been castby the method and the apparatus, characterized in that: gas and powdertrapping caused in molten metal in the process of solidification of themolten metal in a mold can be prevented; cracks on a surface of the castslab caused when the temperature is not uniform can be prevented; andfurther the inner structure of the cast slab can be made fine.

DESCRIPTION OF THE PRIOR ART

[0002] As a method for making a solidification structure to be equi-axedcrystal so that segregation caused in the process of solidification canbe reduced, in continuous casting of steel, electromagnetic stirring iswidely used. For example, this technique is disclosed in JapaneseUnexamined Patent Publication (Kokai) No. 50-23338. It is possible toobtain an equi-axed structure when molten steel in the proximity of asolidification interface is forcibly given a fluidity by electromagneticstirring so that prismatic dendrites can be cut apart. In order toenhance an equi-axed crystal ratio, various investigations have beenmade into the condition of electromagnetic stirring until now andsegregation has been somewhat reduced.

[0003] However, according to the conventional electromagnetic stirringgenerated in a mold, an equi-axed crystal ratio by which a sufficientlyhigh quality of product can be produced is not necessarily obtained inthe case of producing a type of steel (for example, a type of steel, thecarbon content of which is not more than 0.1%) in which it is difficultto form an equi-axed crystal structure. In order to enhance theequi-axed crystal ratio of the above type of steel, in which it isdifficult to form an equi-axed crystal structure, it can be consideredto increase the thrust of electromagnetic stirring generated in a mold.However, when this method is adopted, a surface velocity of molten steelin the mold is increased, and powder trapping is caused on the surfaceof molten steel. As a result, a defect is caused on the surface of theproduct. In some types of steel in which the occurrence of segregationis severely restricted, it is impossible to meet the demand of qualityonly when the equi-axed crystal ratio is enhanced. In these types ofsteel, the grain size of the equi-axed crystal structure must be madefurther fine.

[0004] Conventionally, the following technique is reported, for example,the following technique is disclosed in the United States PatentPublication No. 5722480. Pulse waves, which are generated by turning onand off an electric current, are given in an alternating static magneticfield so that an electromagnetic force directed to the center of a moldside wall is generated. By this electromagnetic force, a lubricatingeffect and a soft contacting effect can be provided. However, accordingto the above method, the electric current is not always made to flow,and an acceleration of the vibrating waves is not controlled. JapaneseUnexamined Patent Publication (Kokai) No. 9-182941 discloses a method inwhich a stirring direction of the electromagnetic stirring isperiodically inverted so that a downward flow cannot be developed anddiffusion of inclusion to a lower portion can be prevented. However,according to this method, vibrating waves are not given onto the frontsolidified shell by a shifting magnetic field. Also, it is not a methodin which acceleration is controlled so that the solidification structurecan be made fine, inclusion can be eliminated for purification and themeniscus can be stabilized.

[0005] Further, according to a method disclosed in Japanese UnexaminedPatent Publication (Kokai) No. 64-71557, an electromagnetic coil forgenerating a magnetic field to rotate molten metal on a horizontalsurface is alternated so that it can exist in a static condition.Therefore, a flow velocity of the meniscus is zero in this method.According to a method disclosed in Japanese Examined Patent Publication(Kokoku) No. 3-44858, in order to prevent V-segregation and porosity ofa cast slab, in an electromagnetic stirring in which a circulationcurrent is caused on a plane perpendicular to a direction in which acast slab is drawn out, a stirring direction is inverted at intervals of10 to 30 seconds. According to a method disclosed in Japanese UnexaminedPatent Publication (Kokai) No. 54-125132, the casting temperature isprescribed for preventing ridging of stainless steel and, in order toprevent positive and negative segregation caused in electromagneticstirring, a ratio of two electric currents, the phases of which aredifferent from each other, is prescribed, and a direction of electriccurrent is switched and an electric current is made to flow in apredetermined direction for 5 to 50 seconds.

[0006] Further, according to Japanese Unexamined Patent Publication(Kokai) No. 60-102263, in order to prevent the occurrence of defectscaused in casting steel of 9%-Ni which is used for a thick plate at lowtemperatures, alternating time of electromagnetic stirring is set at 10to 30 seconds.

[0007] In the above techniques, alternating stirring is conducted in arelatively long period. That is, the above techniques are entirelydifferent from a technique in which vibrating waves are given onto thefront solidified shell by a shifting magnetic field and acceleration ofthe vibrating waves is controlled.

[0008] Therefore, it is desired to develop a new technique by which theabove problems are solved, the solidification structure is made fine,inclusion is purified and further the meniscus is stabilized.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to solve the above problemscaused in the conventional electromagnetic stirring generated in a mold.That is, it is an object of the present invention to provide acontinuous casting method in which vibration is given by a shiftingmagnetic field so that the equi-axed crystal ratio can be enhancedwithout the occurrence of surface defect caused by powder trapping andthe equi-axed crystal structure itself can be further made fine.Further, it is an object of the present invention to provide acontinuous casting apparatus to which the above continuous castingmethod is applied, and also it is an object of the present invention toprovide a cast slab produced by the above method and an apparatus.

[0010] It is another object of the present invention to solve problemscaused in the casting method in which an electromagnetic force is givento molten metal so that solidification of molten metal can be stabilizedand the surface property of a cast slab can be improved.

[0011] The summary for the present invention to accomplish the aboveobjects is described as follows.

[0012] (1) A method for casting molten metal comprising the steps of:pouring molten metal into a mold and solidifying it in the mold whileapplying an electromagnetic force, which is generated by anelectromagnetic coil arranged in proximity to a molten metal pool in themold, upon the molten metal; and vibrating the molten metal, which hasbeen solidified in the mold or is being drawn out downward from the moldwhile being cooled and solidified, by a shifting magnetic fieldgenerated by the electromagnetic coil so that the molten metal isalternately given a high intensity and a low intensity of acceleration.

[0013] (2) A method for casting molten metal comprising the steps of:pouring molten metal into a mold and solidifying it in the mold whileapplying an electromagnetic force, which is generated by anelectromagnetic coil arranged in the proximity of a molten metal pool inthe mold, upon the molten metal; and vibrating the molten metalperiodically, which has been solidified in the mold or is being drawnout downward from the mold while being cooled and solidified, by ashifting magnetic field generated by the electromagnetic coil so thatthe molten metal is alternately given a high intensity and a lowintensity of acceleration.

[0014] (3) A method for casting molten metal comprising the steps of:pouring molten metal into a mold and solidifying it in the mold whileapplying an electromagnetic force, which is generated by anelectromagnetic coil arranged in the proximity of a molten metal pool inthe mold, upon the molten metal; and vibrating the molten metal, whichhas been solidified in the mold or is being drawn out downward from themold while being cooled and solidified, by a shifting magnetic fieldgenerated by the electromagnetic coil so that the molten metal isaccelerated by a high intensity and a low intensity of acceleration in arange not exceeding a predetermined flow velocity when the directionalvectors of high acceleration and low acceleration in the same directionor in the opposite direction are combined with each other.

[0015] (4) A method for casting molten metal comprising the steps of:pouring molten metal into a mold and solidifying it in the mold whileapplying an electromagnetic force, which is generated by anelectromagnetic coil arranged in the proximity of a molten metal pool inthe mold, upon the molten metal; and vibrating the molten metalperiodically in the one direction and the opposite direction, which hasbeen solidified in the mold or is being drawn out downward from the moldwhile being cooled and solidified, by a shifting magnetic fieldgenerated by the electromagnetic coil.

[0016] (5) A method for casting molten metal according to any one ofitems (1) to (4), wherein a process conducted in the mold is a coolingand solidifying process, and also the process conducted in the mold is acontinuous casting process for continuously casting a slab, bloom, slabof medium thickness, or billet.

[0017] (6) A method for casting molten metal according to any one ofitems (1) to (5), wherein a high intensity of acceleration of thevibrating waves in the one direction and the opposite direction is notlower than 10 cm/s² and a low intensity of acceleration of the vibratingwaves in the one direction and the opposite direction is lower than 10cm/s².

[0018] (7) A method for casting molten metal according to item (6),wherein an acceleration and an acceleration time of the vibrating wavesin the one direction, or an acceleration and an acceleration time of thevibrating waves in the opposite direction, and a coefficient ofacceleration time (acceleration×acceleration time) satisfy the followingexpression.

50 cm/s≦coefficient of acceleration time

[0019] (8) A method for casting molten metal according to item (6),wherein an acceleration and an acceleration time of the vibrating wavesin the one direction, or an acceleration and an acceleration time of thevibrating waves in the opposite direction, and a coefficient ofacceleration time (acceleration×acceleration time) satisfy the followingexpressions.

10η≦coefficient of acceleration time

η: viscosity cp of molten metal

[0020] (9) A method for casting molten metal according to item (6),wherein a relation between carbon content C and acceleration satisfiesthe following expressions.

[C]<0.1%: 30 cm/s ²≦acceleration

0.1%≦[C]<0.35%: −80[C]+38 cm/s ²≦acceleration

0.35%≦[C]<0.5%: 133.3[C]−36.7 cm/s ²≦acceleration

0.5%≦[C]: 30 cm/s ²≦acceleration

[0021] (10) A method for casting molten metal according to any one ofitems (1) to (5), wherein an acceleration stop time or an electric powerstop time, the period of which is not more than 0.3 sec and not lessthan 0.03 sec, is provided in the process of acceleration in the onedirection and in the process of acceleration in the opposite direction.

[0022] (11) A method for casting molten metal according to item (6),(7), (8) or (9), wherein an acceleration stop time or an electric powerstop time, the period of which is not more than 0.3 sec and not lessthan 0.03 sec, is provided in the process of acceleration in the onedirection and also in the process of acceleration in the oppositedirection.

[0023] (12) A method for casting molten metal according to item (6),(7), (8) or (9), wherein acceleration is generated for t1, subsequentlya constant flow velocity is kept for t2, next acceleration is generatedin the opposite direction for t3 and thereafter a constant flow velocityis kept for t4 in one period, and molten metal in the mold isperiodically vibrated by repeating this period, and a vibration timet1+t2+t3+t4 in one period is determined to be not less than 0.2 sec andless than 10 sec.

[0024] (13) A method for casting molten metal according to any one ofitems (1) to (8) or item (9), wherein the molten metal is periodicallyvibrated, and a rotating flow in the one direction and the oppositedirection is given to the molten metal.

[0025] (14) A method for casting molten metal according to item (13),characterized in that: when integration is generated for a certainperiod of time, the expression of integrated value of (accelerationtime×acceleration) in the one direction>integrated value of(acceleration time×acceleration) in the opposite direction is satisfied;and an average rotating flow velocity caused by the difference betweenthe integrated values is not more than 1 m/s.

[0026] (15) A method for casting molten metal according to item (13),wherein acceleration of the molten metal in the mold is conducted fort1, subsequently a constant flow velocity is kept for t2, nextacceleration is generated in the opposite direction for t3 andthereafter a constant flow velocity is kept for t4 in one period, moltenmetal in the mold is periodically vibrated by repeating the period, t1ais a time until the vibrating flow velocity becomes zero in time t1, t1bis a time after the vibrating flow velocity becomes zero in time t1, anexpression of t1b+t2>t4+t1a is satisfied, and a rotating flow velocityin one direction caused by the difference in time is not more than 1m/s.

[0027] (16) A method for casting molten metal according to item (13),wherein vibration is periodically given in a period of n cycles, arotating flow is generated by giving acceleration only in apredetermined direction for the rotating time ΔTv after the vibration,and an average rotating flow velocity, number n of cycles and rotatingtime ΔTv satisfy the following expressions.

Average rotating flow velocity≦1 m/s

1≦number n of cycles≦20

0.1≦rotating time ΔTv≦5 sec

[0028] (17) A method for casting molten metal according to item (13),wherein a rotating flow is generated by increasing an acceleration inthe one direction to be larger than an acceleration in the oppositedirection, and an average rotating flow rate is not more than 1 m/s.

[0029] (18) A method for casting molten metal according to item (13),wherein an electric current for rotation generating a rotating flow inone direction is further superimposed on an electric current duringvibration by an electric current of the electromagnetic coil forgenerating a shifting magnetic field so that an average rotating flowvelocity can be not more than 1 m/s.

[0030] (19) A method for casting molten metal according to any one ofitems (1) to (9), wherein the molten metal is periodically vibrated, andvibration of a short period is further added, and the frequency of thevibration of this short period is not less than 100 Hz and not more than30 KHz.

[0031] (20) A method for casting molten metal according to any one ofitems (6) to (9), wherein an electromagnetic coil is arranged in themold or in the proximity of the molten metal pool in the mold whenmolten metal is poured into and solidified in the mold, the molten metalin the mold is periodically vibrated in the one direction and theopposite direction by a shifting magnetic field generated by theelectromagnetic coil, and an electromagnetic brake, which is arranged ina range from the meniscus to a position under the mold distant by 1 m,is applied.

[0032] (21) A method for casting molten metal according to item (11),wherein an electromagnetic coil is arranged in proximity to the moltenmetal pool in the mold when molten metal is poured into and solidifiedin the mold, the molten metal in the mold is periodically vibrated inthe one direction and the opposite direction by a shifting magneticfield generated by the electromagnetic coil, and an electromagneticbrake, which is arranged at a position under the mold distant from themeniscus by 1 m, is applied being synchronized with time at whichacceleration of the electromagnetic coil is stopped in the mold or beingsynchronized with time at which an electric power source is stopped.

[0033] (22) A method for casting molten metal according to any one ofitems (6) to (15), wherein the electromagnetic coil arranged inproximity to the molten metal pool in the mold is arranged in a rangeunder the mold from right below the mold to a position distant from themold by 10 m.

[0034] (23) A method for casting molten metal according to item (22),wherein an electromagnetic brake, which is arranged in a range from aposition above the electromagnetic coil distant by 1 m to a positionbelow the electromagnetic coil distant by 1 m, is applied.

[0035] (24) A method for casting molten metal according to item (11),wherein the electromagnetic coil arranged in proximity to the moltenmetal pool in the mold is arranged in a range from a position rightbelow the mold to a position under the mold distant by 10 m, and theelectromagnetic brake arranged in a range from the meniscus to aposition under the mold distant by 1 m is applied being synchronizedwith the time at which acceleration of the electromagnetic coil isstopped in the mold or being synchronized with the time at which theelectric power source is stopped.

[0036] (25) An electromagnetic coil device used for any one of items (1)to (24), comprising: an electromagnetic drive device for periodicallyvibrating in the one direction and the opposite direction; and a controlunit for controlling the electromagnetic drive device.

[0037] (26) An electromagnetic coil device used for one of items (1) to(24) comprising; an electromagnetic coil; and an electric power sourcefor supplying an electric current to vibrate the electromagnetic coilperiodically in the one direction and the opposite direction or awaveform generating device.

[0038] (27) An electromagnetic coil device used for one of items (1) to(24), comprising: an electromagnetic drive device for vibrating moltenmetal periodically in the one direction and the opposite direction, theelectromagnetic drive device having a function of raising an electriccurrent to a command value in the case of changing a vibratingdirection; and an electric current control device for controlling theelectric current.

[0039] (28) An electromagnetic coil device comprising an electromagneticdrive device, a control device for controlling an electric current, andan electromagnetic brake used in any one of items (1) to (24).

[0040] (29) A cast slab having a negative segregation zone composed of amultilayer structure, the pitch of which is not more than 2 mm and thenumber of the layers of which is not less than three, a dendrite or acrystalline structure zone composed of a deflection structure of amultilayer.

[0041] (30) A cast slab having a negative segregation zone composed of amultilayer structure, the pitch of which is not more than 2 mm and thenumber of the layers of which is not less than three, a dendrite or acrystalline structure zone composed of a deflection structure of amultilayer, wherein the thickness of the negative segregation zone,dendrite or crystalline structure zone is not more than 30 mm.

[0042] (31) A cast slab characterized in that: a corner point (C) of acentral negative segregation line (m) of a negative segregation zone ofan average profile of the negative segregation zone of a multilayerstructure is determined, or a virtual corner point (C′) extrapolatedfrom two adjoining sides of a central segregation line (m) of an arcuatenegative segregation zone is determined; and parallel lines are drawnfrom points (E) on two adjoining sides, which are distant from thecorner point to the inside of the cast slab by 5 mm, to the twoadjoining sides, and a difference between shell thickness D₁ at a pointof intersection (F) with the central segregation line (m) and shellthickness D₂ at the center in the cast slab width direction is not morethan 3 mm.

[0043] (32) A cast slab characterized in that: a corner point of acenter line of dendrite or a crystalline structure zone of deflectionstructure of a multilayer, which has an average profile thereof, isdetermined, or a virtual corner point extrapolated from two adjoiningsides of a center line of the arcuate dendrite or crystalline structurezone is determined; and parallel lines are drawn from points on the twoadjoining sides, which are distant from the corner point to the insideof the cast slab by 5 mm, to two adjoining sides, and a differencebetween shell thickness D₁ at a point of intersection with the centralline and shell thickness D₂ at the center in the cast slab widthdirection is not more than 3 mm.

[0044] (33) A cast slab characterized in that: a shape of the cast slabis circular; and fluctuation of shell thickness at a point on a centralsegregation line (m) of a negative segregation zone of an averageprofile of the negative segregation zone of a multilayer structure isnot more than 3 mm.

[0045] (34) A cast slab characterized in that: a shape of the cast slabis circular; and fluctuation of shell thickness at a point of a centerline of a dendrite or a crystalline structure of an average profile of adendrite structure or a crystalline structure zone of a deflectionstructure of a multilayer is not more than 3 mm.

[0046] (35) A cast slab provided when molten metal is poured into a moldand solidified while an electromagnetic force is applied to the moltenmetal by an electromagnetic coil arranged in the proximity of the moldaccording to item (31) or (33), the cast slab comprising a negativesegregation zone composed of a multilayer structure formed in the innercircumferential direction of the mold having pitch P defined by thefollowing expression (2) in a range of D₀±15 mm in the thicknessdirection with respect to solidified shell thickness D₀ (mm) at the corecenter in the casting direction determined by solidified shell thicknessD (mm) defined by the following expression (1).

D=k(L/V)^(n)  (1)

[0047] D: Solidified shell thickness

[0048] L: Length from meniscus to core center of electromagnetic coil

[0049] V: Rate of casting

[0050] k: Coefficient of solidification

[0051] n: Constant

P=U×t/2  (2)

[0052] U: Rate of solidification (dD/dt (mm/s))

[0053] t: Period of vibration

[0054] (36) A cast slab according to one of items (31) to (35), the castslab having an equi-axed crystal ratio of not less than 50% on theinside of a negative segregation zone composed of a multilayerstructure, on the inside of a dendrite or a crystalline structure zonecomposed of a multilayer-shaped deflection structure.

[0055] (37) A cast slab provided when molten metal is poured into a moldand solidified while an electromagnetic force is given to the moltenmetal by an electromagnetic coil arranged in the proximity of the moldaccording to item (32) or (34), the cast slab comprising a dendrite or acrystalline structure zone, the growing direction of which is regularlydeflected, having pitch P defined by the following expression (2) in arange of D₀±15 mm in the thickness direction with respect to solidifiedshell thickness D₀ (mm) at the core center in the casting directiondetermined by solidified shell thickness D (mm) defined by the followingexpression (1).

D=k(L/V)^(n)  (1)

[0056] D: Solidified shell thickness

[0057] L: Length from meniscus to core center of electromagnetic coil

[0058] V: Rate of casting

[0059] k: Coefficient of solidification

[0060] n: Constant

P=U×t/2  (2)

[0061] U: Rate of solidification (dD/dt (mm/s))

[0062] t: Period of vibration

BRIEF DESCRIPTION OF THE DRAWINGS

[0063]FIG. 1 is a view showing an outline of an arrangement of anelectromagnetic coil in a mold according to the present invention.

[0064]FIG. 2(a) is a diagram for explaining a pattern of an electriccurrent of an electromagnetic coil of the present invention.

[0065]FIG. 2(b) is a diagram for explaining a pattern of a flow velocityof vibration on the front face of solidification.

[0066]FIG. 3 is a graph showing a relation between a period of anelectromagnetic coil current and an equi-axed crystal ratio.

[0067]FIG. 4 is a graph showing a relation between a period of anelectromagnetic coil current and an equivalent diameter of an equi-axedcrystal circle.

[0068]FIG. 5 is a diagram showing an example in which an accelerationstop time is provided, the period of which is not more than 0.3 sec andnot less than 0.03 sec during one direction and the opposite direction.

[0069]FIG. 6 is a diagram showing an example in which an acceleration inthe one direction is 100 cm/s² and an acceleration in the oppositedirection is 50 cm/s².

[0070]FIG. 7 is a schematic illustration showing an outline of thicknessof a solidified shell at a core center in the casting direction of anelectromagnetic coil.

[0071]FIG. 8(a) is a view showing a typical example of a clear corner ofa negative segregation zone of a cast slab of the present invention.

[0072]FIG. 8(b) is a view showing a virtual corner in the case of anunclear negative segregation zone.

[0073]FIG. 9 is a metallograph showing a clear corner of the negativesegregation zone of FIG. 8.

BEST MODE FOR CARRYING OUT THE INVENTION

[0074]FIG. 1 is a view showing the very moment of rotation of moltenmetal in a mold when an electromagnetic field is applied upon the moltenmetal by an electromagnetic coil of the present invention. In thisconnection, reference numeral 1 is an electromagnetic coil, referencenumeral 2 is a side wall on the long side, reference numeral 3 is a sidewall on the short side, and reference numeral 4 is an immersion nozzle.

[0075] The first characteristic of the present invention is not only torotate molten metal in the mold by generating a shifting magnetic fieldby the electromagnetic coil of the mold, but the first characteristic ofthe present invention is also to give an acceleration in the onedirection and the opposite direction to molten metal by a shiftingmagnetic field so that the molten metal can vibrate on the frontsolidified shell. Further, an acceleration of this vibrating waves iscontrolled. The above technique is applied to not only a continuouscasting process but also an ingot process in which a stationary mold isused. In this embodiment, a linear motor is used as the electromagneticcoil. However, the present invention is not limited to the specificembodiment. As long as a shifting magnetic field can be generated, anymagnetic filed generating device may be used, that is, a magnetic fieldgenerating device by which a linear magnetic field is generated is notnecessarily used. For example, a magnetic field generating device bywhich a rotary magnetic field is generated may be used, and any magneticfield generating device by which vibration can be given to molten metalin the one direction and the opposite direction may be used.

[0076] The second characteristic of the present invention is that a loadof the linear motor is increased in the one direction and the oppositerotation and an electric current is continuously supplied, so that aquick rise of the electric current can be accomplished. Due to theforegoing, an electromagnetic force can quickly reach a predeterminedvalue. As a result, it becomes possible to control an acceleration givento molten metal in a wide range.

[0077] According to the above characteristics of the present invention,it is possible to remarkably enhance the inner quality and surfacequality of a cast slab as follows. Instead of rotation of molten metalgenerated by a conventional electromagnetic stirring, vibrating wavesgenerated by a shifting magnetic field is given onto a front solidifiedshell while an acceleration is being controlled in the presentinvention. Due to the foregoing, a prismatic cutting force is increased,so that the solidified structure can be made further finer, and at thesame time, the inner quality of slab can be much purified. Further, achange in the meniscus can be suppressed to as small as possible, thatis, an influence given to the meniscus shape disturbance can besuppressed to as small as possible. In this way, the inner and surfacequality of a cast slab can be remarkably improved.

[0078] In general, a flow velocity of the conventional electromagneticstirring conducted in continuous casting is 20 to 100 cm/s. The presentinventors made investigation into a mechanism of generation of equi-axedcrystals generated by the electromagnetic stirring in the above flowvelocity range. As a result of the investigation, the following weremade clear. Electromagnetic stirring has an effect of inclining a flowof prismatic dendrite onto an upstream side, however, an effect ofcutting a prismatic dendrite apart, which has been conventionallyconsidered to be high until now, is not so high. Instead of the effectof cutting the prismatic dendrite apart, heat transmission between asolidified shell and molten steel is facilitated by the electromagneticstirring. Therefore, overheating of molten steel is reduced, so thatsolidification cores can be easily formed. On the basis of the aboveknowledge, the present inventors made further investigation into amethod by which an effect of cutting the prismatic dendrite apart can bemore remarkably enhanced as compared with the conventional methodwithout impairing an effect of reducing overheat of molten steel whenelectromagnetic stirring is carried out. As a result of theinvestigation, the present inventors found the following. It is veryeffective that an electric current of the electromagnetic coil isperiodically changed as shown in FIG. 2(a), so that vibrating waves,which reciprocate on the front face of solidification, are given tomolten steel. Due to the foregoing, not only the equi-axed crystal ratiocan be enhanced, but also the grain size of equi-axed crystals can bemade fine.

[0079] When an electric current of the electromagnetic coil is changedaccording to the pattern shown in FIG. 2(a), a flow velocity ofvibration on the front solidified shell follows the change in theelectric current as shown in FIG. 2(b), wherein the curve shown in FIG.2(b) becomes a little blunt compared with the curve shown in FIG. 2(a).In a region of t2 or t4 in which the flow velocity of vibration on thefront solidified shell is constant, the vibration flow provides a smalleffect of cutting a prismatic dendrite apart. However, in an onedirection accelerating region t1 and in an opposite directionaccelerating region t3, an acceleration is generated in a vibrating flowon the front solidified shell. Therefore, compared with a rotationalflow of a constant flow velocity, it is possible to give a very strongforce to a prismatic dendrite. By the above effect, it is possible toremarkably enhance an effect of cutting the prismatic dendrite apart.Further, when the vibrating flow velocity on the front solidified shellis made to be the same as that of the conventional method in the regionof t2, it is possible to provide an effect of reducing overheat ofmolten steel by facilitating a heat transmission between thesolidification shell and molten steel. Since a sufficiently strong forceto cut a prismatic dendrite apart is given onto the front solidifiedshell in the accelerating regions t1 and t3, the present invention hasan effect of cleaning by which inclusion is prevented from being caughtby the front solidified shell.

[0080] According to the conventional method, a large quantity ofinclusion is caught by the surface layer of a cast slab, thesolidification rate of which is high, and the degree of purification isdeteriorated. However, according to the present invention, an averageoxygen concentration in a region of 20 mm from the surface of a castslab which was cast according to the present invention can be made lowerthan that of the inner portion of the slab. The rotating flow generatedby the conventional electromagnetic stirring causes the followingproblems. The meniscus goes out of order. When the rotating flowvelocity is increased in order to enhance an equi-axed crystal ratio,powder trapping is caused, and further the rotating flow collides with aside wall on the short side of the mold, so that a strong descendingflow is continuously generated. However, when the vibrating waves, whichreciprocate on the front solidified shell, are given to molten steel, itis possible to prevent the occurrence of disturbance of the meniscus andpowder trapping, and further it is possible to suppress an influence ofthe descending flow. Accordingly, casting can be stably conducted.

[0081] In addition to that, when the rotating flow is superimposed onthe vibrating waves, the purification of inclusion and the generation ofcores can be further facilitated while a shape of the meniscus isstabilized. According to the conventional electromagnetic stirring, anegative segregation zone of solute elements is generated. Therefore, itis impossible to ensure the quality of a cast slab. However, accordingto the present invention, the vibrating waves reciprocate on the frontsolidified shell. Therefore, very thin negative segregation zones of amultilayer structure are generated. Accordingly, the negativesegregation zones are dispersed, and the solidified structure can bemade fine, and at the same time the negative segregation can beprevented.

[0082] As shown in FIGS. 8(a), 8(b) and 9, thin negative segregationzones of a multilayer structure are uniformly generated along the outercircumference of a cast slab at the same distance from the cast slabsurface corresponding to the period of stirring. Accordingly, cracks canbe prevented from proceeding on the cast slab surface, and further theoxidation of a grain boundary can be suppressed. In addition to that, agrowing direction of prismatic crystals (dendrite) in a positivesegregation zone located between the negative segregation zones isalternately changed for each positive segregation zone. Accordingly,compared with a cast slab in which prismatic crystals (dendrite) grow inone direction, the solidification structure is strong with respect tothe occurrence of cracks. For the above reasons, it is possible toproduce a cast slab, the surface layer of which has a highly enhancedfunction, by the casting method of the present invention.

[0083] Next, a coefficient of acceleration time will be explained below.When consideration is given to a material point in a liquid state,concerning a material point movement, it can be said as follows by thelaw of dynamics. “Concerning a momentum of a material point in apredetermined period of time, its change is equal to an impulse of anacting force and a period of time in which the force acts.” Therefore,it is possible to apply the law to a change in the acting force in avibrating condition. That is, (acceleration×acceleration time), which isa coefficient of acceleration time defined by the present invention, canbe used as a parameter of vibration, that is, (acceleration×accelerationtime) can express a change in the impulse or acting force whichrepresents a state of vibration. Due to the foregoing, it is possible tocontrol a state of vibration by adjusting a holding time (t2, t4) in themelting condition and an acceleration giving time (t1, t3) while thecoefficient of acceleration time is used as a parameter.

[0084] In order to provide an effect stably, vibration of the presentinvention, which reciprocates on the front solidified shell, has anappropriate period. An upper and a lower limit of the appropriate periodare determined as follows.

[0085] In order to give an acceleration uniformly in the circumferentialdirection of a cast slab, it is necessary to invert the acceleratingdirection in a period of time in which a boundary layer on the frontsolidified shell is not peeled off. This period of time is shorter than5 seconds and was found by an experiment, and a vibrating time of oneperiod, which will be referred to as a vibrating period hereinafter, isshorter than 10 seconds.

[0086] On the other hand, in order to exhibit the effect of vibration inthe casting direction of a cast slab, it is necessary to give at leastone period of vibration while the cast slab is passing through the coreportion of the electromagnetic coil. At this time, a period of vibrationis not more than a value of (core length)/(casting speed). Therefore,the upper limit of the vibration period is determined by a condition inwhich casting operation can be stabilized in both the circumferentialdirection of the cast slab and the casting direction. The shorter of theperiods becomes the upper limit of the vibration period.

[0087] The present inventors found the following. Molten steel on thefront solidified shell is accelerated in vibration when the condition of(period of vibration)≧2/(frequency of electromagnetic coil) issatisfied. A frequency of the electromagnetic coil for generating ashifting magnetic field is 10 Hz at most. Therefore, a lower limit ofthe period of vibration is not less than 0.2 sec.

[0088] In the present invention, a flow velocity is obtained when adisplacement of a reference point is differentiated by time, and anacceleration is obtained when the flow velocity is differentiated bytime. The acceleration may be obtained when a flow velocity at the pointof time when the flow velocity of vibration is zero is differentiated bytime. Alternatively, the acceleration may be a value of (maximumvibration flow velocity−minimum vibration flow velocity)/t1 or (maximumvibration flow velocity−minimum vibration flow velocity)/t3. Thereference point is located at the center of the long side of the mold orat a point distant from the front solidified shell by 20 mm in front atthe ¼ width. Acceleration time of the coefficient of acceleration timeis t1 or t3 up to the acceleration range t1, in which ti is restrictedby t3. An average rotation flow velocity is an average flow velocityobtained when the acceleration is multiplied by the time and integratedwith respect to the total time and the thus obtained value is averagedwith respect to the time. In FIG. 2, the accelerating region (t1, t3) isa high acceleration time, and the accelerating region (t2, t4), theabsolute value of the acceleration of which is low, is a lowaccelerating region.

[0089] Next, the cast slab of the present invention will be explainedbelow. The first characteristic of the cast slab is that the cast slabhas a negative segregation zone composed of a multilayer structure, thepitch of which is not more than 2 mm and the number of the layers ofwhich is not less than three and that the thickness of the negativesegregation zone is not more than 30 mm. Concerning the negativesegregation zone, there are two cases. One case is shown in FIGS. 8(a)and 9 in which a corner of the negative segregation zone is clear withrespect to a corner of the cast slab, and the other case is shown inFIG. 8(b) in which a corner of the negative segregation zone is notclear with respect to a corner of the cast slab. First, in the caseshown in FIG. 8(a), a corner point (C) of a central negative segregationline (m) is determined in an average profile of a negative segregationzone of a multilayer structure. Parallel lines which are parallel to theadjoining two sides are drawn from points (E) on the adjoining two sidesdistant from the corner point to the inside of the cast slab by 5 mm. Adifference between the shell thickness D₁ at the point of intersection(F) with respect to the negative segregation line (m) and the shellthickness D₂ at the center in the width direction of the cast slab isprescribed to be not more than 3 mm.

[0090] In the case shown in FIG. 8(b), a virtual corner point (C′) isdetermined which is extrapolated from the adjoining two sides of acentral negative segregation line (m) of an arcuate negative segregationzone. Parallel lines which are parallel to the adjoining two sides aredrawn from points (E) on the adjoining two sides distant from the cornerpoint to the inside of the cast slab by 5 mm. A difference between theshell thickness D₁ at the point of intersection (F) with respect to thecentral negative segregation line (m) and the shell thickness D₂ at thecenter in the width direction of the cast slab is prescribed to be notmore than 3 mm.

[0091] In the same manner, a corner point of a center line of a dendriteor a crystalline structure zone of an average profile of the dendrite orthe crystalline structure zone of a deflection structure is determined,or a virtual corner point extrapolated from the adjoining two sides ofthe center line of the arcuate dendrite or the crystalline structurezone is determined, and a prescription is made in the same manner.

[0092] On the other hand, with respect to a circular cast slab,fluctuation of the shell thickness at a point on a central segregationline (m) of a negative segregation zone of a multilayer structure, orfluctuation of the shell thickness at a point on a central segregationline (m) of an average profile of a dendrite of a segregation structureor a crystalline structure zone is prescribed to be not more than 3 mm.

[0093] More specifically, a negative segregation zone of a multilayerstructure, a dendrite of a deflection structure or a crystallinestructure zone is prescribed. That is, concerning the negativesegregation zone, a dendrite of a deflection structure or a crystallinestructure, on the basis of a positional relation shown in FIG. 7, thecast slab comprises a negative segregation zone, a dendrite of adeflection structure or a crystalline structure zone composed of amultilayer structure formed in the inner circumferential direction ofthe mold having pitch P defined by the following expression (2) in arange of D₀±15 mm in the thickness direction with respect to solidifiedshell thickness D₀ (mm) at the core center in the casting directiondetermined by solidified shell thickness D₀ (mm) defined by thefollowing expression (1).

D=k(L/V)^(n)  (1)

[0094] D: Solidified shell thickness

[0095] L: Length from meniscus to core center of electromagnetic coil

[0096] V: Rate of casting

[0097] k: Coefficient of solidification

[0098] n: Constant (0.5 to 1.0)

P=U×t/2  (2)

[0099] U: Rate of solidification (dD/dt (mm/s))

[0100] t: Period of vibration

[0101] In this connection, in the present invention, the installingposition is not limited to a position inside the mold. As long as it isa position in the continuous casting machine and molten steel exists atthe point, the present invention can be applied to any position inprinciple.

[0102] In the present invention, molten metal is not limited to aspecific metal. However, the present invention will be explained herereferring to the appended drawings in which the present invention isapplied to steel.

EXAMPLES Example 1

[0103] In this example, in order to make clear the influence, of avibration pattern which is generated by an electromagnetic coil, on theequi-axed crystal ratio and the grain size of equi-axed crystals, anexperiment was made in which molten steel was poured into a mold havingan electromagnetic coil, the frequency of which was 10 Hz. In thisexperiment, molten steel of 50 kg, the carbon content of which was0.35%, was melted in a high frequency melting furnace and poured into amold made of copper, wherein the width of the mold was 200 mm, thelength was 100 mm and the height was 300 mm. Immediately after themolten steel had been poured into the mold, the molten steel wassolidified while it was being vibrated in the mold by a predeterminedvibrating pattern. After the completion of casting, the ingot was cut ona lateral section, so that the solidified structure was revealedoutside. Then, an area ratio of an equi-axed crystal region (anequi-axed crystal area ratio) and a diameter of an equivalent circle ofthe equi-axed crystal region were evaluated. The vibrating pattern waschanged as follows. In FIG. 2, an electric current of theelectromagnetic coil was set at 100 ampere at maximum and −100 ampere atminimum. Coil current increasing time t1 in which an one directionacceleration is given, coil current decreasing time t3 in which anopposite direction acceleration is given, and minimum coil currentholding time t4 were set at predetermined values. In this way, thevibrating pattern was changed.

[0104]FIG. 3 is a view showing a relation between the period of a changein the coil current (t1+t2+t3+t4) and the equi-axed crystal area ratio.When the vibrating period is reduced, the equi-axed crystal area ratiois increased. However, when the vibrating period becomes shorter than0.2 second, the equi-axed crystal area ratio is suddenly decreased. Thereason why is that the vibrating flow velocity on the front solidifiedshell cannot follow the coil current when the period of the coil currentis decreased. FIG. 4 is a view showing a relation between the period ofthe electromagnetic coil current and the diameter of the equivalentcircle of an equi-axed crystal region. When an absolute value ofacceleration on the front solidified shell (because a value ofacceleration becomes −10 cm/s² in the reverse side accelerating region)is lower than 10 cm/s², the diameter of an equivalent circle of anequi-axed crystal region does not depend upon the vibrating period.Therefore, it is impossible to obtain an effect of making the equi-axedcrystals fine. However, when an absolute value of acceleration on thefront solidified shell becomes a value not less than 10 cm/s², it can beunderstood that the equi-axed crystals are made fine at a vibratingperiod of shorter than 10 seconds. The reason why an effect of makingthe crystals fine can not be obtained except for the above operatingconditions is described as follows. When a value of acceleration of thevibrating flow velocity on the front solidified shell is lower than 10cm/s², a force acting on the prismatic dendrite is weak, so that it isimpossible to obtain an effect of making the crystals fine. When thevibrating period becomes a value not longer than 10 seconds, a boundarylayer is peeled off on the front solidified shell, so that it becomesdifficult for a cutting force generated by acceleration to act on theprismatic dendrite. From the above viewpoint, it can be understood thatthe vibrating condition for making the equi-axed crystals fine is moresevere than the condition for enhancing the equi-axed crystal ratio.

[0105] As a result, the following can be understood. In order to enhancethe equi-axed crystal ratio and make the grain size of the equi-axedcrystals fine, the period of the electromagnetic coil current is set ata value not shorter than 0.2 sec and shorter than 10 sec, and at thesame time, the absolute value of acceleration on the front face ofsolidification is set at a value not less than 10 cm/s².

[0106] In this connection, concerning the acceleration in the presentinvention, the effect depends upon the carbon content of molten steel.In the present invention, the acceleration is restricted as follows.When C≦0.1%, the acceleration is 30 to 300 cm/s². When 0.1%≦C≦0.35%, theacceleration is {80[C]+38} to 300 cm/s². When 0.35%≦C≦0.5%, theacceleration is {133.3[C]−36.7} to 300 cm/s². When 0.5%≦C, theacceleration is 30 to 300 cm/s². The reason why the upper limit is givenhere is that no confirmation was made in the experiment exceeding theabove condition.

[0107] The above knowledge was obtained by the experiment made by thepresent inventors when attention was paid to a relation between theequi-axed crystal ratio and the carbon content.

Example 2

[0108] In this example, a two-strand type continuous casting machine forcontinuously casting billets was used, and cast billets of 120 mm squaremade of carbon steel, the carbon content of which was 0.35%, were castfor 30 minutes at the casting speed of 1.2 m/min. Temperature in atundish was 1530° C. In one of the strands, conventional electromagneticstirring was generated, in which the coil current of the electromagneticstirring device was set at a constant value of 200 ampere and thefrequency was set at 10 Hz, for 30 minutes at the flow velocity of 60cm/s. In the other strand, an electromagnetic coil of the presentinvention capable of giving vibration was arranged in the mold, andmolten steel on the front solidified shell was vibrated under thefollowing conditions. Vibration time of one period of the coil currentwas 2 s (the maximum coil current was 200 ampere, the minimum coilcurrent was −200 ampere, the coil current increasing time was 0.8 s, thecoil current decreasing time was 0.8 s, the maximum coil current holdingtime was 0.2 s, and the minimum coil current holding time was 0.2 s),and acceleration in the one direction and the opposite direction wasgiven under the condition of 50 cm/s² as shown in FIG. 2. After alateral section of the cast billet had been cut and the solidifiedstructure had been revealed, the equi-axed crystal area ratio and thediameter of the equivalent circle of an equi-axed crystal region wereevaluated. Concerning the surface quality of the cast billets, the castslabs were subjected to a visual inspection line, so that each billetwas visually inspected, and the number of defects caused by powder wasinvestigated.

[0109] Concerning the billets on which the conventional electromagneticstirring was conducted, the equi-axed crystal ratio was 30%, and thediameter of the equivalent circle of an equi-axed crystal region was 3.0mm. The flow velocity of molten steel was 60 cm/s, which exceeded acritical flow velocity of powder trapping. Therefore, powder on thesurface of molten steel was trapped, and the defects were caused bypowder, the number of which was 5 pieces/billet. Further, there wasformed a negative segregation zone, the width of which was approximately20 mm, on the surface layer side of the lateral section of the castbillet. On the other hand, when vibration was given by theelectromagnetic coil of the present invention, the equi-axed crystalarea ratio of the cast billet was 50%, and the diameter of theequivalent circle of an equi-axed crystal region was 1.3 mm. Therefore,compared with the conventional electromagnetic stirring, not only theequi-axed crystal area ratio was enhanced, but also the grain size ofthe equi-axed crystals was made fine. Since the molten steel on thefront face of solidification in the mold was vibrated, powder trappingwas not caused, and defects originated from powder were not caused. Onthe lateral face of the cast billet, a negative segregation zone of amultilayer, the pitch of which was 1.5 mm, was formed on the surfacelayer of 15 mm, and also a dendrite of deflection structure of amultilayer was formed.

Example 3

[0110] In this example, a two-strand type continuous casting machine forcontinuously casting slabs was used, and cast pieces of 250 mmthickness×1500 mm width made of carbon steel, the carbon content ofwhich was 0.35%, were cast for 30 minutes at the casting speed of 1.8m/min. Temperature in a tundish was 1550° C. In one of the strands, aconventional electromagnetic stirring was generated, in which the coilcurrent of the electromagnetic stirring device was set at a constantvalue of 500 ampere and the frequency was set at 2 Hz, for 30 minutes atthe flow velocity of 60 cm/s. In the other strand, an electromagneticcoil of the present invention capable of giving stirring was arranged inthe mold. For 15 minutes in the first half of casting, molten steel onthe front face of solidification was vibrated under the followingconditions. Vibrating time of one period of the coil current was 2 s(the maximum coil current was 400 ampere, the minimum coil current was−400 ampere, the coil current increasing time was 0.8 s, the coilcurrent decreasing time was 0.8 s, the maximum coil current holding timewas 0.2 s, and the minimum coil current holding time was 0.2 s), andacceleration in the one direction and the opposite direction was givenunder the condition of 70 cm/s² as shown in FIG. 2. For 15 minutes inthe second half of casting, the molten steel on the front solidifiedshell was vibrated under the following conditions. Vibrating time of oneperiod of the coil current was 2.1 s (the maximum coil current was 400ampere, the minimum coil current was −400 ampere, the coil currentincreasing time was 0.8 s, the coil current decreasing time was 0.8 s,the maximum coil current holding time was 0.2 s, and the minimum coilcurrent holding time was 0.2 s), the acceleration stop time was 0.05 sin the acceleration in the one direction and opposite direction, andacceleration in the one direction and the opposite direction was givenunder the condition of 50 cm/s² as shown in FIG. 5. After a lateralsection of the cast slab had been cut and the solidified structure hadbeen exposed, the equi-axed crystal area ratio and the diameter of theequivalent circle of an equi-axed crystal region were evaluated.Concerning the surface quality of the cast slabs, the cast slabs weresubjected to a visual inspection line, so that each slab was visuallyinspected, and the number of defects caused by powder was investigated.Since vibration marks on the slab surface correspond to a shape of themeniscus, a difference in the levels of the vibration marks wasinvestigated at the same time.

[0111] Concerning the slabs on which the conventional electromagneticvibration was generated, the equi-axed crystal ratio was 30%, and thediameter of the equivalent circle of an equi-axed crystal region was 3.0mm. The flow velocity of molten steel was 60 cm/s, which exceeded acritical flow velocity of powder trapping. Therefore, powder on thesurface of molten steel was trapped, and the defects were caused bypowder, the number of which was 5 pieces/slab. Further, since themeniscus fell into disorder, the difference in the levels of thevibration marks was 3.5 mm. Furthermore, there was formed a negativesegregation zone, the width of which was 20 mm, on the surface layerside of the lateral section of the slab.

[0112] On the other hand, when vibration was given by theelectromagnetic coil of the present invention, irrespective of theexistence of the acceleration stop time, the equi-axed crystal arearatio of the slab was 50%, and the diameter of the circle equivalent tothe equi-axed crystal region was 1.3 mm. Therefore, the equi-axedcrystal area ratio of this example was superior to that of theconventional electromagnetic stirring, and further the grain size of theequi-axed crystals was made fine. Further, since the molten steel on thefront face of solidification in the mold was vibrated, no powdertrapping was caused, and no defects originated from powder were caused.On the lateral section of the cast slab, a negative segregation zone ofa multilayer, the pitch of which was 1.5 mm corresponding to the periodof vibration, was formed on the surface layer of 15 mm, and also adendrite of deflection structure of a multilayer was formed. Concerningthe vibration mark, in the case of a slab in which the acceleration stoptime was not provided, the vibration mark was 5 mm, and in the case of aslab in which the acceleration stop time was provided, the vibrationmark was 3 mm. In both cases, the shape of the meniscus was made uniformcompared with that of the conventional electromagnetic stirring.However, when the acceleration stop time was provided, the meniscus wasmade more uniform. The reason is that a sudden acceleration was reducedwhen the acceleration stop time was provided, so that the meniscus wasmade uniform. In the present invention, the acceleration stop time wasset to be not more than 0.3 sec and not less than 0.03 sec. The reasonis described as follows. When the acceleration stop time is set to bemore than 0.3 sec, an effect of acceleration is deteriorated, and whenthe acceleration stop time is set to be less than 0.03 sec, it becomesimpossible to make the meniscus uniform.

Example 4

[0113] In this example, a two-strand type continuous casting machine forcontinuously casting slabs was used, and cast slabs of 250 mmthickness×1500 mm width made of carbon steel, the carbon content ofwhich was 0.35%, were cast for 30 minutes at the casting speed of 1.8m/min. Temperature in a tundish was 1550° C. In one of the strands, aconventional electromagnetic stirring was conducted, in which the coilcurrent of the electromagnetic stirring device was set at a constantvalue of 500 ampere and the frequency was set at 2 Hz, for 30 minutes atthe flow velocity of 60 cm/s. In the other strand, an electromagneticcoil of the present invention capable of giving vibration was arrangedin the mold. Molten steel on the front face of solidification wasvibrated under the following conditions. Vibrating time of one period ofthe coil current was 2 s (the maximum coil current was 400 ampere, theminimum coil current was −400 ampere, the coil current increasing timewas 0.4 s, the coil current decreasing time was 0.8 s, the maximum coilcurrent holding time was 0.3 s, and the minimum coil current holdingtime was 0.5 s), and acceleration in the normal direction was set at 100cm/s², and acceleration in the opposite direction was set at 50 cm/s² asshown in FIG. 6. After a lateral section of the cast slab had been cutand the solidified structure had been revealed, the equi-axed crystalarea ratio and the diameter of the equivalent circle of an equi-axedcrystal region were evaluated. Concerning the surface quality of thecast slabs, the cast slabs were subjected to a visual inspection line,so that each slab was visually inspected, and the number of defectscaused by powder was investigated. In addition to that, a microscopicexamination was made for checking the number of pieces of inclusion onthe surface layer of the slab.

[0114] Concerning the slabs on which the conventional electromagneticstirring was conducted, the equi-axed crystal ratio was 28%, and thediameter of the equivalent circle of an equi-axed crystal region was 3.1mm. The flow velocity of molten steel was 60 cm/s, which exceeded acritical flow velocity of powder trapping. Therefore, powder on thesurface of molten steel was trapped, and the defects were caused bypowder, the number of which was 6 pieces/slab. Further, there was formeda negative segregation zone, the width of which was approximately 20 mm,on the surface layer side of the lateral section of the cast slab.

[0115] On the other hand, when vibration and rotation according to atime difference in the normal and the reverse direction were given bythe electromagnetic coil of the present invention, the equi-axed crystalarea ratio of the cast slab was 55%, and the diameter of the equivalentcircle of an equi-axed crystal region was 1.3 mm. Therefore, comparedwith the conventional electromagnetic stirring, not only the equi-axedcrystal area ratio was enhanced, but also the grain size of theequi-axed crystals was made fine. Since the molten steel on the frontface of solidification in the mold was vibrated, powder trapping was notcaused, and defects originated from powder were not caused, either. Onthe lateral section of the cast slab, a negative segregation zone of amultilayer, the pitch of which was 1.5 mm, was formed on the surfacelayer of 15 mm, and also a dendrite of deflection structure was formed.When vibration and rotation were simultaneously given to the moltensteel by the electromagnetic coil, the prismatic dendrite was moreeffectively cut apart. Therefore, compared with Example 3 in which onlyvibration was given to the molten steel, the equi-axed crystal ratio wasenhanced in this example. In this connection, when rotation is added tovibration conducted in the molten steel, powder trapping can besuppressed by vibration, however, when a flow velocity of rotationexceeded 1 m/s, powder trapping was caused. Therefore, the flow velocityof rotation was restricted to be not more than 1 m/s.

Example 5

[0116] In this example, a two-strand type continuous casting machine forcontinuously casting slabs was used, and cast slabs of 250 mmthickness×1500 mm width made of carbon steel, the carbon content ofwhich was 0.35%, were cast for 30 minutes at the casting speed of 1.8m/min. Temperature in a tundish was 1550° C. In one of the strands, theconventional electromagnetic stirring was conducted, in which the coilcurrent of the electromagnetic stirring device was set at a constantvalue of 500 ampere and the frequency was set at 2 Hz, for 30 minutes atthe flow velocity of 60 cm/s. In the other strand, the electromagneticcoil of the present invention capable of giving vibration was arrangedin the mold. Molten steel on the front face of solidification wasvibrated under the following conditions. Vibrating time of one period ofthe coil current was 2 s (the maximum coil current was 400 ampere, theminimum coil current was −400 ampere, the coil current increasing timewas 0.8 s, the coil current decreasing time was 0.8 s, the maximum coilcurrent holding time was 0.2 s, and the minimum coil current holdingtime was 0.2 s), and acceleration in the one direction and the oppositedirection was set at 50 cm/s² as shown in FIG. 2. While the molten steelon the front face of solidification was being vibrated, a magnetic forcewas applied upon the molten steel by a static magnetic filed, themagnetic field intensity of which was 3000 gauss, by an electromagneticbrake arranged at a position lower than the meniscus by 1 m. After alateral section of the cast slab had been cut and the solidifiedstructure had been revealed, the equi-axed crystal area ratio and thediameter of the equivalent circle of an equi-axed crystal region wereevaluated. Concerning the surface quality of the cast slabs, the castslabs were subjected to a visual inspection line, so that each slab wasvisually inspected, and the number of defects caused by powder wasinvestigated.

[0117] Concerning the slabs on which the conventional electromagneticstirring was generated, the equi-axed crystal ratio was 31%, and thediameter of the equivalent circle of an equi-axed crystal region was 2.9mm. The flow velocity of molten steel was 60 cm/s, which exceeded acritical flow velocity of powder trapping. Therefore, powder on thesurface of molten steel was trapped, and the defects were caused bypowder, the number of which was 4 pieces/slab. Further, there was formeda negative segregation zone, the width of which was approximately 20 mm,on the surface layer side of the lateral section of the cast slab. Onthe other hand, when vibration was given by the electromagnetic coil ofthe present invention and the electromagnetic brake was applied, theequi-axed crystal area ratio of the cast slab was 56%, and the diameterof the equivalent circle of an equi-axed crystal region was 1.3 mm.Therefore, compared with the conventional electromagnetic stirring, notonly the equi-axed crystal area ratio was enhanced, but also the grainsize of the equi-axed crystals was made fine. Since the molten steel onthe front solidified shell in the mold was vibrated, powder trapping wasnot caused, and defects originated from powder were not caused, either.On the lateral section of the cast slab, a negative segregation zone ofa multilayer, the pitch of which was 1.5 mm, was formed on the surfacelayer of 15 mm, and also a dendrite of deflection structure was formed.When vibration caused by the electromagnetic coil was given togetherwith the electromagnetic brake, the equi-axed crystal ratio was enhancedas compared with that in Example 3 in which only vibration was given.The reason why the equi-axed crystal ratio was enhanced is thatpermeation of molten steel of high temperature into the inside of a castslab was prevented by the electromagnetic brake, and the tesseralcrystal cores, which had been generated by vibration of theelectromagnetic coil, were prevented from being remelted. In thisconnection, when the acceleration stop time is provided in the vibrationgenerated by the electromagnetic coil, it is unnecessary to apply theelectromagnetic brake continuously, that is, it is possible to impressthe electromagnetic brake synchronously with the acceleration stop time.

[0118] Industrial Applicability

[0119] As described above, according to the method of the presentinvention in which the vibration pattern is adjusted by theelectromagnetic coil so as to give vibration to molten metal, it ispossible to give a strong force onto the front solidified shell.Accordingly, compared with the conventional method, not only theequi-axed crystals can be increased but also the grain size of theequi-axed crystals can be made fine. Due to the above effects, it isunnecessary to increase the flow velocity too high for making thesolidified structure fine. Therefore, it is possible to prevent theoccurrence of surface defects caused by powder trapping.

[0120] In this connection, when the present invention is applied to astationary mold, the inner structure of conventional material can beremarkably improved. Accordingly, the productivity and cost can beimproved.

1. A method for casting molten metal comprising the steps of: pouringmolten metal into a mold and solidifying it in the mold while applyingan electromagnetic force, which is generated by an electromagnetic coilarranged in the proximity of a molten metal pool in the mold, upon themolten metal; and vibrating the molten metal, which has been solidifiedin the mold or is being drawn out downward from the mold while beingcooled and solidified, by a shifting magnetic field generated by theelectromagnetic coil so that the molten metal is alternately given ahigh intensity and a low intensity of acceleration.
 2. A method forcasting molten metal comprising the steps of: pouring molten metal intoa mold and solidifying it in the mold while applying an electromagneticforce, which is generated by an electromagnetic coil arranged in theproximity of a molten metal pool in the mold, upon the molten metal; andvibrating the molten metal periodically, which has been solidified inthe mold or is being drawn out downward from the mold while being cooledand solidified, by a shifting magnetic field generated by theelectromagnetic coil so that the molten metal is alternately given ahigh intensity and a low intensity of acceleration.
 3. A method forcasting molten metal comprising the steps of: pouring molten metal intoa mold and solidifying it in the mold while applying an electromagneticforce, which is generated by an electromagnetic coil arranged in theproximity of a molten metal pool in the mold, upon the molten metal; andvibrating the molten metal, which has been solidified in the mold or isbeing drawn out downward from the mold while being cooled andsolidified, by a shifting magnetic field generated by theelectromagnetic coil so that the molten metal is accelerated by a highintensity and a low intensity of acceleration in a range not exceeding apredetermined flow velocity when the directional vectors of highacceleration and low acceleration in the same direction or in theopposite direction are combined with each other.
 4. A method for castingmolten metal comprising the steps of: pouring molten metal into a moldand solidifying it in the mold while applying an electromagnetic force,which is generated by an electromagnetic coil arranged in the proximityof a molten metal pool in the mold, upon the molten metal; and vibratingthe molten metal periodically in the one direction and the oppositedirection, which has been solidified in the mold or is being drawn outdownward from the mold while being cooled and solidified, by a shiftingmagnetic field generated by the electromagnetic coil.
 5. A method forcasting molten metal according to any one of claims 1 to 4, wherein aprocess conducted in the mold is a cooling and solidifying process, andalso the process conducted in the mold is a continuous casting processfor continuously casting a slab, bloom, slab of medium thickness, orbillet.
 6. A method for casting molten metal according to any one ofclaims 1 to 5, wherein a high intensity of acceleration of the vibratingwaves in the one direction and the opposite direction is not lower than10 cm/s² and a low intensity of acceleration of the vibrating waves inthe one direction and the opposite direction is lower than 10 cm/s². 7.A method for casting molten metal according to claim 6, wherein anacceleration and an acceleration time of the vibrating waves in the onedirection, or an acceleration and an acceleration time of the vibratingwaves in the opposite direction, and a coefficient of acceleration time(acceleration×acceleration time) satisfy the following expression. 50cm/s≦coefficient of acceleration time
 8. A method for casting moltenmetal according to claim 6, wherein an acceleration and an accelerationtime of the vibrating waves in the one direction, or an acceleration andan acceleration time of the vibrating waves in the opposite direction,and a coefficient of acceleration time (acceleration×acceleration time)satisfy the following expressions. 10η≦coefficient of accelerationtimeη: viscosity cp of molten metal
 9. A method for casting molten metalaccording to claim 6, wherein a relation between carbon content C andacceleration satisfies the following expressions. [C]<0.1%: 30cm/s²≦acceleration0.1%≦[C]<0.35%: −80[C]+38cm/s²≦acceleration0.35%≦[C]<0.5%: 133.3[C]−36.7cm/s²≦acceleration0.5%≦[C]: 30 cm/s²≦acceleration
 10. A method forcasting molten metal according to any one of claims 1 to 5, wherein anacceleration stop time or an electric power stop time, the period ofwhich is not more than 0.3 sec and not less than 0.03 sec, is providedin the process of acceleration in the one direction and in the processof acceleration in the opposite direction.
 11. A method for castingmolten metal according to claim 6, 7, 8 or 9, wherein an accelerationstop time or an electric power stop time, the period of which is notmore than 0.3 sec and not less than 0.03 sec, is provided in the processof acceleration in the one direction and also in the process ofacceleration in the opposite direction.
 12. A method for casting moltenmetal according to claim 6, 7, 8 or 9, wherein acceleration is generatedfor t1, subsequently a constant flow velocity is kept for t2, nextacceleration is generated in the opposite direction for t3 andthereafter a constant flow velocity is kept for t4 in one period, andmolten metal in the mold is periodically vibrated by repeating thisperiod, and a vibration time t1+t2+t3+t4 in one period is determined tobe not less than 0.2 sec and less than 10 sec.
 13. A method for castingmolten metal according to any one of claims 1 to 8 or claim 9, whereinthe molten metal is periodically vibrated, and a rotating flow in theone direction and the opposite direction is given to the molten metal.14. A method for casting molten metal according to claim 13,characterized in that: when integration is generated for a certainperiod of time, the expression of integrated value of (accelerationtime×acceleration) in the one direction>integrated value of(acceleration time×acceleration) in the opposite direction is satisfied;and an average rotating flow velocity caused by the difference betweenthe integrated values is not more than 1 m/s.
 15. A method for castingmolten metal according to claim 13, wherein acceleration of the moltenmetal in the mold is generated for t1, subsequently a constant flowvelocity is kept for t2, next acceleration is generated in the oppositedirection for t3 and thereafter a constant flow velocity is kept for t4in one period, molten metal in the mold is periodically vibrated byrepeating the period, t1a is a time until the vibrating flow velocitybecomes zero in time t1, t1b is a time after the vibrating flow velocitybecomes zero in time t1, an expression of t1b+t2>t4+t1a is satisfied,and a rotating flow velocity in one direction caused by the differencein time is not more than 1 m/s.
 16. A method for casting molten metalaccording to claim 13, wherein vibration is periodically given in aperiod of n cycles, a rotating flow is generated by giving accelerationonly in a predetermined direction for the rotating time ΔTv after thevibration, and an average rotating flow velocity, number n of cycles androtating time ΔTv satisfy the following expressions. Average rotatingflow velocity≦1 m/s1≦number n of cycles≦200.1≦rotating time ΔTv≦5 sec17. A method for casting molten metal according to claim 13, wherein arotating flow is generated by increasing an acceleration in the onedirection to be larger than an acceleration in the opposite direction,and an average rotating flow rate is not more than 1 m/s.
 18. A methodfor casting molten metal according to claim 13, wherein an electriccurrent for rotation generating a rotating flow in one direction isfurther superimposed an electric current during vibration by an electriccurrent of the electromagnetic coil for generating a shifting magneticfield so that an average rotating flow velocity can be not more than 1m/s.
 19. A method for casting molten metal according to any one ofclaims 1 to 9, wherein the molten metal is periodically vibrated, andvibration of a short period is further added, and the frequency of thevibration of this short period is not less than 100 Hz and not more than30 KHz.
 20. A method for casting molten metal according to any one ofclaims 6 to 9, wherein an electromagnetic coil is arranged in the moldor in the proximity of the molten metal pool in the mold when moltenmetal is poured into and solidified in the mold, the molten metal in themold is periodically vibrated in the one direction and the oppositedirection by a shifting magnetic field generated by the electromagneticcoil, and an electromagnetic brake, which is arranged in a range fromthe meniscus to a position under the mold distant by 1 m, is applied.21. A method for casting molten metal according to claim 11, wherein anelectromagnetic coil is arranged in the proximity to the molten metalpool in the mold when molten metal is poured into and solidified in themold, the molten metal in the mold is periodically vibrated in the onedirection and the opposite direction by a shifting magnetic fieldgenerated by the electromagnetic coil, and an electromagnetic brake,which is arranged at a position under the mold distant from the meniscusby 1 m, is applied being synchronized with time at which acceleration ofthe electromagnetic coil is stopped in the mold or being synchronizedwith time at which an electric power source is stopped.
 22. A method forcasting molten metal according to any one of claims 6 to 15, wherein theelectromagnetic coil arranged in proximity to the molten metal pool inthe mold is arranged in a range under the mold from right below the moldto a position distant from the mold by 10 m.
 23. A method for castingmolten metal according to claim 22, wherein an electromagnetic brake,which is arranged in a range from a position above the electromagneticcoil distant by 1 m to a position below the electromagnetic coil distantby 1 m, is applied.
 24. A method for casting molten metal according toclaim 11, wherein the electromagnetic coil arranged in proximity to themolten metal pool in the mold is arranged in a range from a positionright below the mold to a position under the mold distant by 10 m, andthe electromagnetic brake arranged in a range from the meniscus to aposition under the mold distant by 1 m is applied being synchronizedwith the time at which acceleration of the electromagnetic coil isstopped in the mold or being synchronized with the time at which theelectric power source is stopped.
 25. An electromagnetic coil deviceused for any one of claims 1 to 24, comprising: an electromagnetic drivedevice for periodically vibrating in the one direction and the oppositedirection; and a control unit for controlling the electromagnetic drivedevice.
 26. An electromagnetic coil device used for any one of claims 1to 24 comprising; an electromagnetic coil; and an electric power sourcefor supplying an electric current to vibrate the electromagnetic coilperiodically in the one direction and the opposite direction or awaveform generating device.
 27. An electromagnetic coil device used forany one of claims 1 to 24, comprising: an electromagnetic drive devicefor vibrating molten metal periodically in the one direction and theopposite direction, the electromagnetic drive device having a functionof raising an electric current to a command value in the case ofchanging a vibrating direction; and an electric current control devicefor controlling the electric current.
 28. An electromagnetic coil devicecomprising an electromagnetic drive device, a control device forcontrolling an electric current, and an electromagnetic brake used inany one of claims 1 to
 24. 29. A cast slab having a negative segregationzone composed of a multilayer structure, the pitch of which is not morethan 2 mm and the number of the layers of which is not less than three,a dendrite or a crystalline structure zone composed of a deflectionstructure of a multilayer.
 30. A cast slab having a negative segregationzone composed of a multilayer structure, the pitch of which is not morethan 2 mm and the number of the layers of which is not less than three,a dendrite or a crystalline structure zone composed of a deflectionstructure of a multilayer, wherein the thickness of the negativesegregation zone, dendrite or crystalline structure zone is not morethan 30 mm.
 31. A cast slab characterized in that: a corner point (C) ofa central negative segregation line (m) of a negative segregation zoneof an average profile of the negative segregation zone of a multilayerstructure is determined, or a virtual corner point (C′) extrapolatedfrom two adjoining sides of a central segregation line (m) of an arcuatenegative segregation zone is determined; and parallel lines are drawnfrom points (E) on two adjoining sides, which are distant from thecorner point to the inside of the cast slab by 5 mm, to the twoadjoining sides, and a difference between shell thickness D₁ at a pointof intersection (F) with the central segregation line (m) and shellthickness D₂ at the center in the cast slab width direction is not morethan 3 mm.
 32. A cast slab characterized in that: a corner point of acenter line of dendrite or a crystalline structure zone of deflectionstructure of a multilayer, which has an average profile thereof, isdetermined, or a virtual corner point extrapolated from two adjoiningsides of a center line of the arcuate dendrite or crystalline structurezone is determined; and parallel lines are drawn from points on the twoadjoining sides, which are distant from the corner point to the insideof the cast slab by 5 mm, to two adjoining sides, and a differencebetween shell thickness D₁ at a point of intersection with the centralline and shell thickness D₂ at the center in the cast slab widthdirection is not more than 3 mm.
 33. A cast slab characterized in that:a shape of the cast slab is circular; and fluctuation of shell thicknessat a point on a central segregation line (m) of a negative segregationzone of an average profile of the negative segregation zone of amultilayer structure is not more than 3 mm.
 34. A cast slabcharacterized in that: a shape of the cast slab is circular; andfluctuation of shell thickness at a point of a center line of a dendriteor a crystalline structure of an average profile of a dendrite structureor a crystalline structure zone of a deflection structure of amultilayer is not more than 3 mm.
 35. A cast slab provided when moltenmetal is poured into a mold and solidified while an electromagneticforce is applied to the molten metal by an electromagnetic coil arrangedin the proximity of the mold according to claim 31 or 33, the cast slabcomprising a negative segregation zone composed of a multilayerstructure formed in the inner circumferential direction of the moldhaving pitch P defined by the following expression (2) in a range ofD₀±15 mm in the thickness direction with respect to solidified shellthickness D₀ (mm) at the core center in the casting direction determinedby solidified shell thickness D (mm) defined by the following expression(1). D=k(L/V)^(n)  (1)D: Solidified shell thickness L: Length frommeniscus to core center of electromagnetic coil V: Rate of casting k:Coefficient of solidification n: Constant P=U×t/2  (2)U: Rate ofsolidification (dD/dt (mm/s)) t: Period of vibration
 36. A cast slabaccording to one of claims 31 to 35, the cast slab having an equi-axedcrystal ratio of not less than 50% on the inside of a negativesegregation zone composed of a multilayer structure, on the inside of adendrite or a crystalline structure zone composed of a multilayer-shapeddeflection structure.
 37. A cast slab provided when molten metal ispoured into a mold and solidified while an electromagnetic force isgiven to the molten metal by an electromagnetic coil arranged in theproximity of the mold according to claim 32 or 34, the cast slabcomprising a dendrite or a crystalline structure zone, the growingdirection of which is regularly deflected, having pitch P defined by thefollowing expression (2) in a range of D₀±15 mm in the thicknessdirection with respect to solidified shell thickness D₀ (mm) at the corecenter in the casting direction determined by solidified shell thicknessD (mm) defined by the following expression (1). D=k(L/V)^(n)  (1)D:Solidified shell thickness L: Length from meniscus to core center ofelectromagnetic coil V: Rate of casting k: Coefficient of solidificationn: Constant P=U×t/2  (2)U: Rate of solidification (dD/dt (mm/s)) t:Period of vibration